During the nineteenth century, biologists discovered that in the absence of air, cells produce ethanol or lactic acid, whereas in the presence of air, cells consume O2 and produce CO2 and H2O. In 1937 the German-born British biochemist Hans A. Krebs reported the discovery of the citric acid cycle—also called the tricarboxylic acid cycle or Krebs cycle. The elucidation of the citric acid cycle not only explained how pyruvate is broken down to CO2 and H2O; it also highlighted the key concept of cycles in metabolic pathways. For his discovery, Hans Krebs was awarded the Nobel Prize in physiology and medicine in 1953.
Because the citric acid cycle is localized in the matrix of mitochondria, we will begin with a general description of mitochondrial structure and function, knowledge obtained mainly through experiments on isolated mitochondria (see Web Topic 11.1). We will then review the steps of the citric acid cycle, emphasizing the features that are specific to plants. For all plant-specific properties, we will consider how they affect respiratory function.
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